Here’s a true story.
A few years ago hub was working on our water system and said water would be off for a few hours. (Our system is a well and we fill up large tanks of water periodically from the well.)
I knew that anything DH embarks upon always takes much longer than estimated, so I filled up every available vessel in the house in the short amount of time I had. What with buckets, jugs, large pots and so on, I had just about 53 gallons of water.
We used that water very sparingly - I could wash no dishes, cooked very simply (for instance, used the water that I washed the rice with for washing hands, and then for flushing the toilet, etc), we did minimal “bucket baths” or sponge baths, flushed only for solid stuff.
After three days the water came back on, I had about 2 quarts of water left, and we and the house were filthy.
So, 40 gallons would be bare survival for a few days. You won’t have enough to really clean anything, bathe, flush the toilet more than once or twice a day if that, do any real cooking, no washing dishes, etc.
If that’s a good level of survival - at most, 3 days of extremely minimal water use, fine.
I like to have much, much more leeway than that. BTDT.
Where possible, an old fashioned hand pump in the yard can be decorative and functional. It didn’t kill grandma and grandpa and it won’t kill me. In some places county government provides testing of well water for a small fee.
If that’s a good level of survival - at most, 3 days of extremely minimal water use, fine.
IF you've got a few minutes warning, here's a $20 way to store 100 gallons for a short period. Remember, don't have all your eggs- or water- in one basket.